1. Field of the Invention
Aspects of the present invention relate to a directional crystallization method of directly irradiating a laser beam having a specific width and length on amorphous silicon to crystallize the amorphous silicon, and more particularly, to a method of crystallizing amorphous silicon into polycrystalline silicon having a grain boundary parallel to a crystal growth direction without using a mask and having excellent surface roughness, a method of fabricating a thin film transistor using the same, and an organic light emitting diode (OLED) display device including the thin film transistor.
Aspects of the present invention also relate to a flat panel display in which a directional crystallization method and a super grained silicon (SGS) crystallization method are used to crystallize an amorphous silicon layer and a method of fabricating the same, and a semiconductor device and a method of fabricating the same. More particularly, aspects of the present invention relate to a flat panel display device having excellent characteristics, which includes a thin film transistor simultaneously having high electron mobility and uniformity of characteristics by forming semiconductor layers in peripheral and pixel regions using directional crystallization using a laser and SGS crystallization, respectively, a semiconductor device, and methods of fabricating the same.
2. Description of the Related Art
Generally, polycrystalline silicon layers are widely used as semiconductor layers for thin film transistors because they have high field effect mobility, and can be applied to high-speed operation circuits and constitute CMOS circuits. The thin film transistors using these polycrystalline silicon layers are usually applied to active devices of active-matrix liquid crystal display (AMLCD) devices and switching and driving devices of OLED display devices.
Methods of crystallizing the amorphous silicon layer into the polycrystalline silicon layer include solid phase crystallization (SPC), metal induced crystallization (MIC), metal induced lateral crystallization (MILC), excimer laser crystallization (ELC), and sequential lateral solidification (SLS).
Among these, a method of crystallizing using a laser can be performed at a low temperature, and thus, low temperature polycrystalline silicon (LTPS) may be fabricated.
Excimer laser annealing (ELA) is used to fabricate a thin film transistor using such a low temperature polycrystalline silicon layer. In ELA, amorphous silicon is melted and crystallized for a short time of 30 to 200 ns, and thus, ELA does not damage a glass substrate. The actual duration is only several hundred nanoseconds per pulse, and ELA can be used at room temperature. ELA uses multiple pulses applied in a length of 15 to 30 cm and a width of 0.2 to 3 mm. A beam profile of a laser, the number of pulses, initial substrate temperature, and a deposition condition and method of an amorphous silicon layer are critical factors which affect the final crystallinity of the resulting polycrystalline silicon layer.
Recently, flat panel display devices, such as a liquid crystal display (LCD) device, an organic light emitting diode (OLED) display device, or a plasma display panel (PDP), have attracted attention as such resolve disadvantages of conventional, heavy, and large display devices, such as a cathode ray tube. In the flat panel display device, such as the OLED display device or the LCD device, a thin film transistor is widely used as a switching device, and a semiconductor layer of the thin film transistor is formed of polycrystalline silicon.
The polycrystalline silicon layer may be formed by as-deposition, which is a method of directly depositing a polycrystalline silicon layer on a glass substrate at a temperature of 580° C. or more and under a pressure of 0.1 to 0.2 torr. However, in such a method, common glass substrates cannot endure such a high temperature for the time necessary for crystallization of the silicon layer, and thus a large glass panel cannot be used. It is noted that the method has been successful at 530° C. using silane (SiH4) gas, which, however, is difficult to commercialize.
Second, to form the polycrystalline silicon layer, a solid phase crystallization (SPC) method may be used, which is the most direct and oldest method for obtaining a polycrystalline silicon thin film from amorphous silicon. In the SPC method, silicon ions are injected into a deposited amorphous silicon layer, and then annealed at 600° C. or less for at least several tens of hours. The size of the final grains depends on a dose of injected silicon ions, annealing temperature, and annealing time. The polycrystalline silicon layer obtained by such SPC method generally has several μm grains, which is relatively large, but also has many defects in a corresponding grain. Such defects are known to negatively affect performance of the resulting thin film transistor.
Third, to form the polycrystalline silicon layer, a rapid thermal annealing (RTA) method may be used, which has high mass-productivity and is performed at a temperature of 700 to 1100° C. for several seconds. Fewer defects in the grain are generated than with the SPC method, but deformation of or damage to the substrate during annealing is a decisive problem. Though the annealing time is short, fine contraction or swelling of a panel or circuit results in a misalignment margin of a pattern, and thus the process becomes impossible. Recently, such an RTA process is widely used as a dehydrogenation process of removing hydrogen atoms after depositing an amorphous silicon layer using a plasma enhanced chemical vapor deposition (PECVD) method or a process of activating ions after ion injection rather than a process of crystallizing an amorphous silicon layer.
Like ELA above, there is a sequential lateral solidification (SLS) method of crystallization using a mask. However, the SLS has disadvantages of being a complicated process and costly because a primary grain boundary interrupting current flow is formed and the laser is irradiated using a mask in growing the crystals.
However, although the above-described crystallization methods have advantages and disadvantages, there is no method that produces a polycrystalline layer simultaneously having high electron mobility in a peripheral region and uniformity in a pixel region, which are needed in an organic light emitting diode (OLED) display device.
In the SLS process, silicon crystalline particles may be laterally grown to a predetermined length by appropriately controlling an intensity of laser energy, an irradiation range of a laser beam, and a translation distance, thereby crystallizing amorphous silicon into a single crystalline level. An irradiation device used in the SLS process concentrates the beam in a small region using a mask such that an amorphous silicon layer stacked on a large substrate may not be simultaneously converted into a polycrystalline state. Thus, to irradiate the laser beam on the entire region of the substrate, after the substrate on which the amorphous silicon layer is stacked is located on a stage, the laser beam is applied to an area, and then the substrate moves to apply the laser beam to another area. However, the “primary grain boundary” and “secondary grain boundary” formed by the SLS process affect electrical characteristics of a resulting thin film transistor, and thus variations of characteristics occur in the thin film transistor using polycrystalline silicon having such grain boundaries.
Also, when the crystallization is performed by controlling a translation range of the laser beam irradiated to remove the “primary grain boundary” to less than ½ of the laser beam width, new grain boundaries are formed by collision or division of the grains as the grains grow. The new grain boundary is irregularly formed so that the overall grain boundaries are non-uniform. Such non-uniformity of the grain boundary affects the electrical characteristics of the thin film transistor to cause variations in the characteristics of the thin film transistor, and thus defects are generated by the concentration of an electric field at a protrusion created due to the non-uniform primary grain boundary, and a mura phenomenon occurs. Also, the use of a mask makes the process more complicated and expensive.